The present invention relates to a continuous process for production of moisture-curable polyurethane sealants and adhesives. The process is achieved using raw or preblended ingredients fed to a twin-screw extruder, but does not require the additional production resources involved in processing a prepolymer (i.e. pre-mixed reactive components).
Manufacturing polyurethane materials using a continuous process is not novel. The concept is covered in various articles, textbooks and patents. The current literature, however, is limited to processes that either include a pre-polymer manufacturing step (see U.S. Pat. Nos. 5,905,133 and 5,795,948), have a different polyurethane chemistry than moisture-cure sealants/adhesives with resultant performance characteristics that classify them as thermoplastics, elastomers or foams (see U.S. Pat. Nos. 6,624,278, 6,623,676, 6,040,381, 5,908,701, 4,661,531, 4,250,292, 3,963,679, and 3,642,964) or both (see U.S. Pat. Nos. 6,916,390, 6,294,637, 5,905,133, 5,795,948, 5,037,864, 4,879,322, 4,857,565, 4,742,095, and 4,342,847). The novelty of the proposed process lies in the use of a single machine to mix and react the moisture-curing formulation in-situ.
Polyurethanes are formed through the reaction of a multi-functional (functionality >1.0) polyol (—OH) with a multi-functional (functionality >1.0) isocyanate (—NCO). There are many applications and types of polyurethanes (elastomer, foam, thermoplastic, etc.), but this invention relates particularly to polyurethanes used as sealants and adhesives. In these formulations, the NCO/OH molar ratio can range from 1.5 to 3.0 in order to achieve the proper performance characteristics and maintain their liquid and/or thermoplastic state. In addition to these reactive components, most polyurethane adhesive formulations utilize ingredients that include but are not limited to fillers, tackifiers, plasticizers, antioxidants, catalysts, desiccants, pigments and viscosity modifiers.
The most common manufacturing method for polyurethane sealants and adhesives is to charge the raw materials, either manually or through an automated handling system, into a high-intensity batch mixer. Often a two-step or moisture-curable prepolymer approach is used. In the first step the polyol and isocyanate are pre-mixed in the desired ratio, possibly with several other ingredients, to form the prepolymer. In the second step this prepolymer is mixed with the remaining raw materials to achieve the final formulation. The prepolymer is typically formulated so that it can be used in several different finished products. As a result, while both production steps may occur in the same mixer, often a large quantity of the prepolymer is manufactured in the mixer for the first step then transferred to one or more secondary mixers to make several finished batches.
As an alternative to the prepolymer process, a one-step manufacturing approach may also be used. In the one-step approach, all of the raw materials are sequentially charged (again, either manually or through an automated raw material handling system) to a single, high-intensity batch mixer. In this approach the order of addition of the ingredients is important in order control the finished product quality. Once the product is complete it is transferred directly to the final packaging form and there is no need for a secondary mixing step.
While efficient mixing of the raw materials through energy transfer from the mixer is a primary requirement for adequate product quality, there are several other important process requirements for these formulations. Since the final product is moisture-curable, control of water in the process is critical. Moisture finds its way into the product either as a natural part of the raw materials as provided from the supplier or through exposure to environmental conditions, such as absorption of the moisture from humid air into the raw materials and/or finished product. For this reason, vacuum and/or chemical desiccation are used to remove excess moisture in the product and prevent premature curing in the packaging container. The application of vacuum in the mixing process is also used to remove the carbon dioxide gas, which is a natural by-product of the water and Isocyanate reaction.
Process temperature is another important control variable in manufacturing these materials. In either the single or two-step approach product temperatures can run from 125° F. to 250° F. depending on the nature of the raw materials and their thermal stability. These temperatures are selected to drive the urethane reaction (with or without the presence of a catalyst) and/or promote mixing depending on the stage of the process. High shear mixing blades are frequently used, which add heat to the process. Batch temperature is typically maintained by means of a heating and/or cooling medium (oil, water, etc.) that is circulated through an internal or external enclosure on the mixing vessel (coil, jacket, etc.) to add or remove heat depending on the process requirements.
Typical commercial process times depend on variables such as the formulation, raw material feed rates, scale of the equipment, manpower limitations, and heat transfer capabilities. In the two-step approach, standard batch times for each step can range from 2 to 6 hours. In a one-step approach, batch times may run between 4 to 12 hours.
Once the mixing process is complete the fully formulated product is transferred to the packaging line. To package the product, pressure is applied that allows the material to be injected into the container (typically cartridges, sausage-packs/chubs, pails or drums) for delivery to the customer.
The complexity of the polyurethane formulations and process make the batch manufacturing approach outlined above inefficient and undesirable. Several factors combine that cause the process to be labor and/or capital intensive:    a. The number of raw materials in the formulation and the limitations related to their order of addition requires a significant amount of automated material handling equipment or dedicated labor to manually charge the ingredients. When labor is used to fill this need, undesirable safety issues may be introduced such as operator exposure to chemical and/or ergonomic hazards. In some cases, manufacturers use a combination of automated equipment and labor to process the materials, depending on the nature of the raw material supply (bulk vs. non-bulk).    b. Product quality requirements often necessitate in-process quality testing to ensure critical product parameters, including but not limited to moisture content, are within specifications to proceed to the next process step. This is accomplished either through on-line instrumentation or by manually sampling the batch in the middle of the process. The former requires a high degree of capital investment and sophistication. The latter involves batch delays as samples are taken by the operator and brought to plant's QC laboratory for evaluation.    c. By their nature, moisture-curable polyurethanes quickly and easily build up and cure on equipment surfaces, particularly once the product is transferred and machinery is exposed to the moisture in the ambient air. Keeping equipment surfaces clean is a constant battle and cleaning must occur on a regular basis either to prevent contamination (for example from a cured piece of polyurethane coming loose in a subsequent batch or in the case of a product color change) or maintain equipment performance (such as keeping agitator blades clean for sufficient mixing). As a result, a fair amount of time and energy is often invested in cleaning the equipment. This is not only an issue in terms of the operator time invested in cleaning, but also creates the undesirable environmental and economic consequences related to solvent disposal.
In addition to the negative cost impact of the factors outlined above, there are additional productivity limitations associated with the batch process that further affect the manufacturing economics. These include but are not limited to:    a. The low surface area/volume ratio present in batch mixers. As the size/scale of batch mixers increases, there is more volume of the product in the mixer and less surface area (per unit volume) that is in contact with the walls (for heat transfer) or the exposed to the vacuum in the headspace (for mass transfer of moisture and/or carbon dioxide as outlined above). Since heat and mass are only transferred at these interfaces, the poor surface area/volume ratio in batch mixers will adversely affect batch times. The result is lower throughput and higher manufacturing costs per pound for the finished product.    b. In-process testing. If on-line instruments are not installed, the equipment must be shut down occasionally to check the quality. Idle time on the equipment while the sample is taken, brought to the lab and evaluated reduces the production capacity for the equipment.    c. Equipment cleaning. As outlined above, manufacturing moisture-curable sealants and adhesives requires frequent equipment cleaning. Since the equipment sits idle whenever it is being cleaned, this further reduces the production capacity and increases manufacturing costs.
These limitations combine to make the batch process unattractive from a capital or operational cost perspective, and possibly both. There are significant economic, safety, environmental and productivity gains that can be realized from an alternative production method that addresses the concerns outlined above.